(a) field of invention
The present invention relates to an insulator housing made from polymeric materials.
(b) brief description of the prior art
Electrical insulator housings for outdoor use require certain properties in order to function efficiently. For instance, such housings must provide a creepage (leakage) path greater than overall housing length in order to reduce surface electrical stress across the housing. The higher the voltage across the conductor to be insulated, the longer the creepage path must be in order to prevent flashover (short circuiting).
In polluted areas, a longer creepage path for a given voltage is needed because the surface resistance of the insulator is often lowered by deposits from the air. As a result, the unprotected insulator surface, when wetted by rain, fog or condensation, may become conducting.
For the highest DC voltages currently employed, use of conventional bushing material (porcelain) can be impractical and uneconomical owing to the size of insulator needed (see TECHNICAL PROBLEMS ASSOCIATED WITH DEVELOPING HVDC CONVERTER STATIONS FOR VOLTAGES ABOVE 600 kV by P.C.S. KRISHNAYYA et al, IEEE Transaction on Power Delivery, Jan. 1987, vol. PWRD-2, No. 1, p. 174).
As noted above, conventional bushing housings are prone to flashover due to accumulated pollution on the porcelain sheds. Such housings are subject to aerodynamically-deposited pollution, and also, especially for insulators energized with direct voltage, to electrostaticallydeposited pollution because of the lack of adequate external stress control over their porcelain sheds.
Furthermore, large conventional housing designs suffer from water cascading effects in severe weather, thereby short circuiting the insulation between sheds. In addition, the conventional housings, because of their unitary nature, cannot be altered or adjusted to cope with changing conditions, and if damaged, have to be completely replaced.
The large porcelain bushings of the prior art are also cumbersome, heavy and fragile, requiring their complete production in a factory before transportation to the site of use. Use of porcelain also incurs manufacturing limitations, on the extent to which desirable features, such as long creepage path with well spaced sheds, can be combined.
Various attempts to solve these problems have been made. For example, greasing the insulator surface to increase its hydrophobicity and regular washing of the insulator surface to remove pollution deposits have been tried. Neither method has proved totally successful.
In a paper entitled BUSINGS WITH SILICON RUBBER SHEDS by F. HAMMER & J. WELTGEN, Paper No. 44.09 delivered at the Fourth International Symposium on High Voltage Engineering (Athens, Greece 5-9 Sept. 1983), a hollow composite insulator comprising an inner tube made of glass-fibre reinforced with epoxy resin and an outer sheath comprising silicone rubber sheds, is described.
Although these silicone rubber sheds better inhibit flash-over, the external profile of the bushing described in this paper is substantially that of the conventional porcelain insulators and therefore suffers from many of the disadvantages discussed above for porcelain insulators.
C.H.A. Ely et al, in a paper entitled THE BOOSTER SHED: PREVENTION OF FLASHOVER OF POLLUTED SUBSTATION INSULATORS IN HEAVY WETTING, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-97, No. 6, Nov/Dec 1978, disclose the use of supplementary sheds to deflect rain from the more vulnerable parts of insulators. In this paper, skirts of plastic are fixed between the sheds of a conventional insulator so that each skirt overhangs and thereby protects from precipitation the porcelain sheds attached thereunder. The critical amount of pollution deposit, causing insulator flashover at working voltage, in increased when using these skirts or "booster sheds", by a factor of about 4 or 5, i.e. housings with booster sheds can sustain more pollution before flashover occurs.
However, use of booster sheds may actually increase pollution deposits (by preventing rain washing) thereby causing flashover voltage to be undesirably low.
L. Gion et al in a paper entitled NEW INSULATORS WITH HELICODAL SHEDS FOR LIES AND HIGH VOLTAGE APPARATUS, delivered at the Conference Internationale des Grands Reseaux Electriques a Haute Tension, Paris, 15-25th June 1960, disclose how use of helicoidal sheds allows production of shorter insulators of equal efficacy to larger, classical insulators, by taking advantage of the auto-cleaning properties and increased leakage line (=path) inherent in the helicoidal geometry.
However, the insulators discussed in this paper are made of conventional materials (ceramic and porcelain) and therefore still suffer the material deficiencies discussed above.